We use ro/di water to ensure that there are no dissolved solids entering the water column. Then to ensure there are no dissolved solids going in, we use TDS meters to measure the level, wanting it to be 0 of course, to tell us when it is time to change our filters since the resin will change colors to tell us when it is time for new.
If it is just RO or just DI water, it likely isn't at 0 and can be a source for our nutrient issues. Since getting married, I lost my kitchen. This is where I had my ro/di unit set up and have been stuck with distilled water from walmart ever since. Nitrates directly into my reef tank through water changes and top offs.